docs: def print_help_fields():
      """ Print help about fields and field formatters.
      """
      # Mock entries, so they fulfill the expectations towards a field definition
      def custom_manifold():
          "named rTorrent custom attribute, e.g. 'custom_completion_target'"
          return ("custom_KEY", custom_manifold)
      def kind_manifold():
          "file types that contribute at least N% to the item's total size"
          return ("kind_N", kind_manifold)
  
      print('')
      print("Fields are:")
      print("\n".join(["  %-21s %s" % (name, field.__doc__)
          for name, field in sorted(engine.FieldDefinition.FIELDS.items() + [
              custom_manifold(), kind_manifold(),
          ])
      ]))
  
      print('')
      print("Format specifiers are:")
      print("\n".join(["  %-21s %s" % (name, doc)
          for name, doc in sorted(formatting.OutputMapping.formatter_help())
      ]))
      print('')
      print("Append format specifiers using a '.' to field names in '-o' lists,\n"
            "e.g. 'size.sz' or 'completed.raw.delta'.")